Films of poly(dimethylsiloxane) deposited on silanized silicon wafers are unstable and dewet by nucleation and growth of dry patches. We have monitored the growth of holes : (i) In thick films (that is, for film thicknesses, e, larger than 10 mu m), holes open at constant velocity. The radius of the dry patch, R, grows linearly with time, t. (ii) In microscopic films (e < 1 mu m), we find R(t) similar to t(0.66+/-0.08). We interpret this result by a slippage of the polymer chains on the solid surface.